1. Field of the Invention
This invention relates to wireless data communication systems and specifically to setting thresholds of an enhanced medium access control function.
2. Description of Related Art
With a view to obviate the need for wired cabling connections between stations in local area networks (LANs), wireless local area networks have been developed, and are now commercially available. These wireless local area networks employ a plurality of mobile network stations, which may be data processing devices (such as personal computers) having wireless communication capability.
A wireless local area network includes an access point, which serves as a base station, and a plurality of other network stations. The network stations within a group or a cell communicate directly to their corresponding access point. This access point forwards messages to the destination station within the same cell or through a wired distribution system to a wired destination station (e.g., server, printer, etc.) or to another access point, from which such messages arrive finally at a wireless destination station.
Wireless LANs present several challenges not encountered by wired based LANs. For example, the size of the cell served by an access point must be established and the potential mobility of network stations has to be taken into account. Also, for wireless based networks, which use a single channel, it is substantially more difficult to detect collisions due to the large dynamic range of receive signal levels. Therefore, wireless local area networks typically employ a collision avoidance scheme, instead of collision detection.
These issues are dealt with by setting certain communication parameters. One parameter is referred to as the carrier detect threshold for receiving a desired signal. The carrier detect threshold is the level of the observed carrier signal, below which a network station or access point will not attempt to process data signals. For example, by varying the carrier detect threshold, it is possible to select the signal level above which signals are received and processed. A second parameter is referred to as the defer threshold. The defer threshold is the level of the observed carrier signal, above which a network station will defer the transmission of data signals.
Wireless local area networks, LANs, are generally configured based on a medium access control (MAC) arrangement that employs a listen-before-talk scheme like CSMA/CA (carrier sense multiple access with collision avoidance) as described by the IEEE 802.11 standard.
In accordance with the medium access control (MAC) arrangement, each local area network station begins transmission when it determines that no other station is transmitting communication signals. To this end, each station defers its transmission of signals, so long as the signal level it receives from other stations is above the defer threshold. Thus, the medium access control (MAC) arrangement prevents a second station remotely located from the first station, to start signal transmission that overlaps in time with an earlier started transmission by the first station typically, the second station defers its signal transmission for a randomly selected period of time.
A third parameter includes a collection of thresholds referred to as roaming thresholds. A first roaming threshold called a cell search threshold sets the receive signal strength or signal-to-noise ratio (SNR) below which a network station will scan for other access points and determine, based on the results of the scan, whether the network station should communicate via a different access point. A second roaming threshold called an out-of-range threshold sets the receive signal strength or SNR below which the network station concludes the access point serving the network station is out of range. In this instance, the network station will perform a more robust scan for new access points with which to communicate.
When establishing or modifying a wireless LAN a network administrator sets or adjusts the above-described parameters. This process is time consuming, and relies on the administrator to optimize the system.